1. Field of the Invention
The present invention relates to a crystallization apparatus, and the crystallization apparatus uses light rays on amorphous or polycrystalline semiconductor thin film, so as to melt and crystallize the amorphous or polycrystalline semiconductor thin film. More particularly, the present invention relates to a crystallization technology, i.e., phase modulated excimer laser annealing (PMELA), and in the crystallization technology, a laser beam having a light intensity distribution obtained by phase modulation is irradiated on a non-single crystal semiconductor thin film, so as to crystallize the non-single crystal semiconductor thin film.
2. Description of Related Art
A technology of crystallizing a non-crystallized semiconductor layer formed on an insulator such as a glass substrate, so as to obtain a crystallized semiconductor layer and form a thin film transistor (TFT) with the crystallized semiconductor layer as an active layer, is known.
For example, in an active matrix liquid crystal display (LCD) apparatus, a semiconductor film such as a silicon film is disposed, a TFT is formed on the glass substrate, and the TFT is used as a switching device for switching display.
The step of forming the TFT includes the step of crystallizing non-single crystal semiconductor thin film such as amorphous or polycrystalline semiconductor thin film. For the crystallization technology, for example, laser crystallization technology is well known, and in the laser crystallization technology, a short pulse laser beam with high energy is used to melt the irradiated region of the non-single crystal semiconductor thin film and to crystallize the irradiated region.
Recently, laser crystallization apparatus used for production adopts a laser beam of lengthwise light beam (e.g. 500 μm*300 mm) with uniform intensity distribution to irradiate the amorphous silicon. However, for this method, the grain size of the obtained semiconductor film is smaller than or equal to 0.5 μm, and the grain size is relatively small. Therefore, a grain boundary exists in a channel region of TFT, so the performance of the TFT has limits, for example, the characteristics of the TFT are restrained.
In order to improve the performance of the TFT, a technology for manufacturing high quality semiconductor film having large grain is required. For the crystallization method satisfying the requirement, in various laser crystallization technologies, the following technology is particularly concerned. An excimer laser beam with the light intensity distribution of inverted wave peak pattern formed after phase modulation is irradiated to the non-single crystal semiconductor thin film, so as to crystallize the non-single crystal semiconductor thin film.
The method of irradiating the laser beam with the uniform intensity distribution to the amorphous silicon without performing the phase modulation is referred to as an excimer laser annealing (ELA) technology. The technology of performing crystallization after irradiating the excimer laser beam which is phase-modulated is referred to as PMELA technology. The excimer laser beam with specified light intensity distribution is irradiated to the non-single crystal semiconductor thin film, such that the irradiated portion of the semiconductor film is melted, and the irradiated portion of the semiconductor film is crystallized.
A light modulation device such as a phase modulation device is used, for example, a phase modulation device such as a phase shifter is used to perform phase modulation for the incident laser beam, so as to obtain the excimer laser beam with a specified light intensity distribution. The non-single crystal semiconductor thin film is, for example, amorphous silicon thin film or polycrystalline silicon thin film formed on the glass substrate.
In recently developed the PMELA technology, the excimer laser beam is irradiated once to melt and crystallize the region with a size of several millimeters square. Through the crystallization process of the non-single crystal semiconductor thin film, a crystallized silicon thin film (for example, please refer to non-patent document 1) having relatively same grains and size of approximately several microns to 10 microns is formed. The TFT manufactured on the crystallized silicon thin film formed by the method has excellent electrical characteristics.
The PMELA crystallization technology has the following excellent characteristics. That is, the using efficiency of the laser beam is high, such that it is possible to obtain crystals of grains with large grain diameter. However, in order to obtain stable electrical characteristics, it is necessary to accurately position the grains. In order to crystallize the semiconductor film of a large area, an irradiation technique called the step-and-repeat irradiation is used. According to the step-and-repeat irradiation technique, the following steps are performed repeatedly. That is, after the non-single crystal semiconductor film is irradiated with the laser beam, the glass substrate is moved to the next irradiating position and stops. Then, the non-single crystal semiconductor film is irradiated with the laser beam again.
In the PMELA crystallization technology, in order to evaluate the melting and the crystallizing states in a micro-region of the silicon thin film, it is required to observe the micro-region. For optically observing the crystallization process, the following technology is provided. That is, the illumination optical system used for observation is disposed to observe after the pulse energy light beam is irradiated (for example, please refer to patent document 1).
FIG. 8 is a view of a construction example of a crystallization apparatus for crystallizing by using PMELA. In FIG. 8, the crystallization apparatus 100 has an optical system 101 used for crystallization. The optical system 101 applied for crystallization forms light pattern used to perform the crystallization of grains with large grain diameter. The optical system 101 used for crystallization includes a laser beam source 111, a beam expander 112, a homogenizer 113, a phase shifter (e.g., a phase modulation device) 114, an imaging optical system 115, and a stage 140. The stage 140 guides the substrate 130 to a pre-specified position. The beam expander 112 expands the laser beam from the laser beam source 111, and the homogenizer 113 homogenizes the light intensity in the plane of the laser beam. Then, the laser beam is irradiated to the phase shifter 114. The excimer laser beam passing through the phase shifter 114 is modulated to become a specified light intensity distribution, and is irradiated to the substrate 130 by the imaging optical system 115.
Also, with respect to the ELA technology of irradiating the laser beam having a uniform intensity distribution onto the amorphous silicon without performing the phase modulation, the following technology is provided. The light irradiating onto the ultraviolet (UV) region is used as the energy light beam for crystallization, and the light irradiating onto the visible light range is used as the illumination light for observation, so as to optically observe the crystallization process.
In FIG. 8, an observation system 120 used to observe the processed region during the crystallization includes an illumination optical system used for observation and a microscopy-observation optical system. The illumination optical system used for observation includes an illumination light source 121 used for observation, a beam expander 122, a half mirror 123, and an annular mirror system 124. The microscopy-observation optical system includes a microscopy-optical imaging system 125, a light detector 126, and a photography apparatus (for example, please refer to patent document 2).
In order to promote crystal growth by heating under a uniform temperature distribution to form relatively large grains, reduce the crystallization defect, and improve the electrical characteristics of the semiconductor film, a technology of irradiating a first energy light beam and a second energy light beam is provided. The first energy light beam causes the semiconductor film to crystallize. Further, the absorption rate of the second energy light beam in the semiconductor film is smaller than the absorption rate of the first energy light beam, and the energy of the second energy light beam is smaller than the energy required for the semiconductor film to crystallize. The second energy light beam reaches till the lower part of the semiconductor film and the substrate, and heats the semiconductor film in the thickness direction of the semiconductor film, so as to heat the substrate, and to reduce temperature difference before and after the irradiation of the first energy light beam. The fact that the excimer laser beam serves as the first energy light beam, and the light including the wavelength component of the visible light range serves as the second energy light beam is disclosed (please refer to patent documents 3 and 4).
In order to maintain the temperature distribution of the heat-processed substrate uniform, it is already known that a cover layer structure is disposed on the upper layer part of the substrate. For the processed substrate, for example, a processed film such as a semiconductor thin film is formed on a basic substrate spaced apart by an insulation film, and a cover film composed of an insulation film is disposed on the processed film. The cover film is used to reduce the heat generated by the heated processed substrate, so as to homogenize the temperature distribution of the processed substrate.
In order to position the grain with high accuracy, it is necessary to accurately project the pattern of the light modulation device on the substrate. But, if the laser beam is continuously irradiated frequently and repeatedly, the environmental temperature of the optical system may rise or heat expansion may occur on the lens system. Therefore, when the laser beam is irradiated, the projection magnification of the pattern, projected onto the substrate, of the light modulation device changes.
It can be ensured through simulation that when a laser with a wavelength of 308 nm is used, if the environmental temperature rises by 3° C., for example, as a telecentric lens with a minification of ⅕ is used to reduce the pattern of 10 millimeters square of the light modulation device to an area of 2 millimeters square so as to be transferred onto the substrate, and the magnification is changed from 1/5.000 to 1/4.994. Thus, an error of approximately 2.4 μm is generated on the periphery of the projection pattern.
In the crystallization region formed by the irradiation of the laser beam, if a transistor is formed across the grain boundary in the subsequent steps, the change of the projection magnification may degrade the switch characteristics.
In the PMELA crystallization technology capable of forming quasi single crystal of several microns, the light modulation device plays an important role in the crystal growth. An important factor of the PMELA crystallization technology is optimizing the shape of the light modulation device. The light intensity distribution of the light modulation device is transferred onto the processed substrate by the excimer laser.
[Non-patent document] Kohki Inoue, Mitsuru Nakada, and Masakiyo Matsumura; Journal of the Institute of Electronic, Information and Communication Engineers Vol.J85-C,N0.8, pp. 624-629, 2002 “Amplitude and Phase Modulated Excimer-Laser Melt-Regrowth Method for Silicon Thin-Films—A New Growth Method of 2-D Position-Controlled Big Grains”
[Patent document 1] Japan patent publication NO. 2006-66462
[Patent document 2] Japan patent publication NO. 2005-294801
[Patent document 3] Japan patent publication NO. 2000-68520
[Patent document 4] Japan patent publication NO. 2002-261015
As described above, in the PMELA crystallization technology, the light modulation device plays an importance role in the crystal growth. An important factor of the PMELA crystallization technology is optimizing the shape of the light modulation device. The light intensity distribution of the light modulation device is transferred onto the processed substrate by the excimer laser.
However, the excimer laser is an UV light, and is a laser irradiating in the form of a pulse. Hence, it is difficult to observe the light intensity distribution formed on the substrate. Therefore, it is impossible to optimize the light modulation device, which is used for obtaining the required crystallization growth.
For the PMELA crystallization technology, patent document 1 has proposed that the optical observation is performed on the crystallized state. For the ELA technology, patent document 2 has proposed that the excimer laser in the UV range is used to perform the crystallization, and the light of the visible light range is used for observation. In the two documents, the states after crystallization are observed. Hence, in order to optimize the shape of the light modulation device according to the observation result, it is necessary to perform in advance the crystallization in the region for the monitoring. After the crystallization state is observed, it is necessary to adjust the light modulation device. Therefore, it is impossible to instantly adjust the light modulation device, and it is impossible to adjust the light modulation device when the processed part is crystallizing.
Also, in order to obtain the uniform temperature distribution, the energy light beam irradiated to the processed region is divided into the first energy light beam for crystallization, and the second energy light beam for heating. The light with smaller absorption rate and smaller energy is used as the second energy light beam for heating (0106, 0115, and 0117 paragraphs in patent documents 3 and 4). Therefore, it is possible to perform the heating without affecting the processed region.
Therefore, the recently provided technologies have the following problems. That is, it is impossible to observe and determine the light intensity distribution of the energy light beam for crystallization under the state of performing the crystallization, and it is impossible to adjust the light modulation device or the metal aperture according to this observation and determination, so as to optimize the light intensity distribution.
The inventor of the application proposed a visualizing apparatus in which light in a visible range is used for the light intensity distribution pattern transferred to the processed substrate, so as to visualize the light intensity distribution. The visualizing apparatus provides a visualization of the light intensity distribution, so as to adjust the light modulation device or a metal aperture forming the light intensity distribution, and to align the position of the optical axis of the light performing the crystallization with the irradiated region.
In the present invention, two kinds of laser beams, i.e., the laser beam in UV range and the laser beam of visible light range, are used as the laser beams for irradiating the processed substrate. The laser beam in UV range is uniformly irradiated to the processed substrate, such that the crystallized region in the processed substrate melts. In the other aspect, the laser beam of visible light range has the energy required by crystallization and has patterned light intensity distribution, such that the light intensity distribution overlaps with the melted region, and the crystal growth is performed in the melted region.
The absorption rate of an amorphous silicon film for the laser beam of visible light range is small. If the laser beam of visible light range having the patterned light intensity distribution is only irradiated on the amorphous silicon film, the absorption rate of the amorphous silicon film is small; thus, it is impossible to crystallize the amorphous silicon film. Therefore, the inventor focuses on the fact that the absorption rate of the melted amorphous silicon film for the laser beam in the visible range being quite large, then the crystallization and the visualization may be achieved at the same time by irradiating the laser beam of visible light range having the patterned light intensity distribution onto the melted amorphous silicon film.
Within the time corresponding to the pulse irradiating time (about 30 nsec) of the excimer laser, the light source of the laser beam in the visible range must apply an energy density (critical fluence) required for damaging a film of the processed substrate and commencing the crystallization. The critical fluence is, for example, an energy density of 100 mJ/cm2.
Therefore, in the crystallization apparatus, in order to use the visible light to perform the crystallization, the visible light capable of obtaining high energy density output is required. If the visible light source is provided, the crystallization and the visualization are realized at the same time.